That’s because the rock record is very imperfect. a lot of environmental conditions (e.g. usually water, no oxygen, no predators, quick sedimentation etc) have to be met for a dead animal to become a fossil. Human fossils are a very good example – there must have been millions of Homo erectus and there are no more than two dozen fossil skeletons preserved!
Great question – I think one for Ceri! My understanding is that fossilisation is a fairly random event, and you will never achieve an accurate record of any one slice through time, let alone at regular intervals through time! It takes a specific set of conditions for it to work properly. Ceri – I hope this isn’t too basic… 🙂
Ashleighlou you’re asking some very interesting questions! Firstly, there are many transitional fossils that document the relationships between fish and amphibians, amphibians and reptiles, dinosaurs and birds. However, the fossil record s very, very incomplete. Not all dead organisms stay around long enough to be fossilised. This is because their tissues are broken down by bacterial decay or other animals feeding off the dead tissue. Usually, organisms with hard parts- bones or shells- stand a better chance of being fossilised because these tissues take much longer to break down. However, this means that a lot of the soft bodied organisms that would have existed throughout history (we know jellyfish existed 500 million years ago because we’ve found their fossilised embryos!) won’t have been fossilised. So, gaps in the fossil record do not mean that there were no organisms- it means they may not have been fossilised, or been discovered by us yet! There’s a lot of rock out there we’ve never searched for fossils in!